A conventional seat belt retractor has a generally cylindrical spool on which seat belt webbing is wound. The spool is mounted to be rotatable about its longitudinal axis to wind in or pay out seat belt webbing, depending upon the direction of rotation. The spool is biased in the wind-in direction by a clock spring so that the seat belt maintains a relatively low level force on the vehicle occupant tending to pull him back into the vehicle seat in a comfortable manner but allowing him to move forward, for example to adjust a vehicle radio or access a storage compartment in the vehicle. A vehicle crash activates a crash sensor that locks the spool against rotation to prevent forward motion of the vehicle occupant.
Modern seat belts employ a load limiting function because it was found that sudden locking of the seat belt retractor, especially in a high speed vehicle crash, could exert excessive force to the vehicle occupant. To reduce this phenomenon modern seat belt retractors allow some controlled payout of the belt webbing before locking fully.
One known load limiting device, especially effective during the initial moments of a crash, functions by interposing a plastically or elastically deformable member in the force path. For example, a torsion bar may be incorporated into the retractor spool. A torsion bar is made of steel that twists about its own axis when high torque is applied and can rotate up to 7 or 8 times while still remaining intact to reduce the crash forces felt by the vehicle occupant.
To provide load limiting that more closely matches the varying forces exerted on the vehicle occupant during a crash, especially the higher forces during the initial moments of the crash, a two-stage load limiting seat belt retractor can be used. These so-called digressive load limiting seat belt retractors create a higher level of load limiting for a predetermined time or distance. During the initial stages of a crash a first stage of load limiting involves both the torsion bar twisting and an additional load limiting device, for example a wire being either bent or twisted or pulled out of a housing in the spool, to absorb the crash forces. When the wire is fully bent, twisted or extended (depending on the mechanism employed) then a second stage of load limiting, at a lower level, involves only the torsion bar.
However, distortion of a wire element in known seat belt retractors has been found to provide a non-uniform load limiting effect. An increase in load occurs for a short time during the initial bending of the wire element and also during the transition from the first stage to the second stage of load limiting. The present invention provides a seat belt retractor that can deliver a more constant level of load limiting.